An Illinois RSU tax calculator estimates the federal + 4.95% Illinois flat tax owed on each vest. Default employer withholding (22% federal, no state supplemental) usually under-collects — the calculator below shows the dollar shortfall for your bracket.
Calculate RSU Withholding
Estimate your RSU tax withholding and net proceeds after vesting.
Try Calculator →Open the Illinois RSU tax calculator with these inputs prefilled →
How Illinois taxes your RSU vest
Illinois treats RSU income exactly like ordinary W-2 wages: when shares vest, the fair-market value on the vest date is added to your Form W-2 Box 1 in the year of vest, taxed at the flat 4.95% Illinois individual income tax rate, and reported alongside federal withholding (IRS Publication 525). Because Illinois uses a flat tax — not progressive brackets — your Illinois liability scales linearly with RSU income; the bracket you fall into doesn't matter on the state side, only the dollar amount of the vest does.
The two numbers that matter on every vest:
- Federal supplemental withholding rate: 22% on RSU income up to $1M (IRS Publication 15). Above $1M, the rate jumps to 37%.
- Illinois supplemental withholding rate: 4.95% flat. Illinois does not publish a separate "supplemental" rate the way California or New York do — employers withhold at the same 4.95% rate that applies to all Illinois wage income, so the default withholding and your actual state liability are usually identical on the state side.
If your marginal federal bracket is higher than the 22% supplemental rate, your employer is under-withholding on the federal side — and you owe the difference at filing time. The Illinois piece typically washes, but the federal gap shows up on every IL vest.
Worked examples (Illinois, 2025)
These three scenarios use the EquityTax calculator with the inputs shown. The math is identical to what you'll see when you click through to the calculator.
Example 1 — Junior IC, single filer
Inputs: $125K salary, single filer, 100 RSUs vesting at $85 FMV ($8,500 RSU income).
Result:
- Federal withholding: $1,870
- Illinois withholding: $420.75
- Estimated total tax on the vest: $3,111
- Shortfall vs default withholding: $170
The 22% federal default closely matches the 24% federal marginal rate at this income level, so the gap is small — $170. Illinois is a flat tax, so default state withholding (4.95% × $8,500 = $420.75) equals the actual Illinois liability on the vest. No state shortfall.
Run this scenario in the calculator →
Example 2 — Mid-level IC, married filing jointly
Inputs: $205K salary, married filing jointly, 400 RSUs vesting at $125 FMV ($50,000 RSU income).
Result:
- Federal withholding: $11,000
- Illinois withholding: $2,475
- Estimated total tax on the vest: $14,581
- Shortfall vs default withholding: $336
Total household income is $255K, which puts this couple in the 24% federal bracket. The 22% supplemental rate is close but still short by 2 percentage points on the RSU income. Illinois again contributes a clean $2,475 (4.95% × $50,000), with no state-side gap.
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Example 3 — Senior IC, married filing jointly
Inputs: $355K salary, married filing jointly, 1,500 RSUs vesting at $205 FMV ($307,500 RSU income).
Result:
- Federal withholding: $67,650
- Illinois withholding: $15,221.25
- Estimated total tax on the vest: $119,058
- Shortfall vs default withholding: $28,960.50
This is where Illinois RSU earners get hurt. Total household income lands at $662,500, deep into the 35% federal bracket. The employer withholds at 22%, so on $307,500 of RSU income the federal gap alone is $28,960.50. Illinois state tax is a clean $15,221.25 (flat 4.95%), and there's also an estimated $2,027.24 underpayment penalty if no quarterly estimate is made. Total owed at filing: $30,987.74.
Run this scenario in the calculator →
Why employer withholding usually isn't enough in Illinois
The gap is almost entirely federal, not state. Illinois's flat 4.95% means employer supplemental withholding lines up with actual state liability for every vest size, every filing status, every income level. The federal supplemental rate, by contrast, is a fixed 22% — and the moment your total income crosses into the 24%, 32%, 35%, or 37% brackets, your employer is under-withholding on RSU income.
The fix is straightforward and depends on the size of the gap:
- Adjust your W-4 so your paycheck withholding picks up the difference across the year (IRS W-4 instructions).
- Make a quarterly estimated payment if the shortfall is concentrated around a single vest date.
- Sell-to-cover at vest — if your broker offers it, this can absorb the federal gap automatically; the state gap (4.95%) usually doesn't need separate action.
The 22% federal supplemental rate is a default — not your actual marginal rate. If your total income (salary + RSU) puts you in the 32%, 35%, or 37% bracket, the federal under-withholding alone can dwarf your Illinois state liability.
How the Illinois RSU tax calculator handles state and federal
The Illinois calculator uses Illinois's actual 2025 flat 4.95% rate from the Illinois Department of Revenue, not a generic state placeholder. Because Illinois has no progressive brackets, the state portion of the engine is a straight multiplication on vest-day RSU income — but the federal side still walks the full 2025 IRS bracket table for your filing status, which is where the real shortfall comes from.
Open the calculator with your inputs →
FAQ
Does Illinois charge a higher rate on RSU income than on regular salary? No. Illinois uses a single flat 4.95% rate on all individual income, including RSU vesting income. There is no separate supplemental rate or surtax on equity compensation at the state level.
Why is my Illinois RSU shortfall almost always zero? Because Illinois withholding (4.95% flat) equals Illinois liability (4.95% flat) on the RSU income. The shortfall you see on Illinois RSU vests is almost entirely federal — driven by the 22% supplemental rate being lower than your real marginal federal bracket.
Do I owe Illinois tax on RSUs that vest while I live out of state? Generally only on the portion attributable to Illinois workdays during the vesting period — sourcing rules apply. Verify your specific situation with the Illinois Department of Revenue and a CPA.
When do I need to make a quarterly estimated payment in Illinois? If you expect to owe more than $1,000 at filing after withholding and credits, the IRS safe-harbor rules generally require estimated payments. For most Illinois RSU earners, the federal gap is the trigger — the 4.95% state piece is already covered by default withholding.
Sources
- IRS Publication 525, Taxable and Nontaxable Income — RSU treatment, fair-market-value rules.
- IRS Publication 15, Employer's Tax Guide — federal supplemental withholding rate (22% / 37%).
- Illinois Revenue Department — state flat-tax rate and equity-comp guidance.
- EquityTax Illinois RSU Calculator (internal engine, last verified 2026-05-09).
Tax Disclaimer: This content is for educational purposes only. Always consult with a licensed tax professional or certified public accountant before making financial decisions related to equity compensation, tax planning, or investment strategies.
Estimate only — not financial or tax advice. Consult a qualified CPA before making decisions about exercising stock options, selling equity, or other financial moves.